1. Field of the Invention
The present invention relates to polyvinyl alcohol (hereinafter abbreviated as PVA) fibers of good absorbency, and to a nonwoven fabric comprising them.
2. Description of the Prior Art
In the past, polyacrylates have been typically used in the preparation of high-absorbent fibers. Based on their characteristics, they are widely used in various fields of sanitary materials, medical goods, electromechanical materials, food-wrapping materials, agricultural materials, construction materials, and the like. However, the high-absorbent fibers of this type have some problems in that they are weak by themselves and therefore can not substantially placed into practical use when alone, and, in addition, their workability is not good and they are expensive.
As to fibers prepared from PVA, the hydroxyl groups in the PVA molecules form intramolecular and intermolecular hydrogen bonds and the bonds are firm enough to prevent intramolecular and intermolecular water penetration. In water at room temperature, therefore, no change in the morphology of the fibers is found and they absorb little water. Given this situation, various studies have been conducted with the objective of making such PVA fibers highly absorbent. For example, one approach is to mix spin a highly-absorbent resin with PVA as discussed in JP-A 1-192815, which discloses that when a highly-absorbent polymer prepared by introducing a crosslinking structure into an alkali metal salt of a copolymer of an xcex1-olefin or a vinyl compound with maleic anhydride is spun with PVA by mix spinning the materials, highly-absorbent PVA fibers result. However, in the method of production described in the patent publication, a blend of PVA with a highly-absorbent polymer that does not form fibers by itself is used and therefore the strength of the fibers produced is low, that is, lower than 1 cN/dtex. Another problem with the method is that the crosslinking reaction time for heat treatment is long and the running cost is therefore high.
On the other hand, for example, JP-A 3-014613 discloses that dry spinning of a carboxylic acid-modified PVA gives PVA fibers having a water absorption of 100 times by weight or more. However, since the degree of carboxylic acid modification of PVA of these fibers is high, that is, from 9 to 15 mol %, the costs of the PVA fibers are high. Another problem with the method is that, because the properties of the fibers are not good, the fibers often present problems in working them into fibrous structures such as nonwoven fabrics. JP-A 7-189023 discloses examples of spinning a self-crosslinkable PVA polymer or introducing a crosslinking structure into non-self-crosslinkable PVA fibers to make the fibers absorbent. By this method, however, the draw ratio of the fibers can not be increased up to 3 times or more, and therefore the strength of the fibers is low. In addition, because the crystallinity of the PVA polymer is high, the water absorption of the fibers is approximately 1 time and is low. Further, because no catalyst is used in manufacture of the fibers, the crosslinking reaction takes a long time and the running cost is therefore high.
On the other hand, some ordinary water-soluble PVA fibers prepared from PVA that has a low degree of hydrolysis or is copolymerized with a hydrophilic group may swell in water at room temperature, but their water absorption is less by a factor of than 10 times. Accordingly, these fibers can not be high-absorbent fibers, and therefore can not be used for the manufacture of nonwoven fabrics that are required to have a high water absorption.
As so mentioned hereinabove, producing PVA fibers of high absorbency presents problems in that the absorbency of the fibers produced is low, the productivity of the fibers is low and the production costs are high, and when nonwoven fabrics comprising the fibers are produced, the physical properties such as strength and elongation of the fibers are unsatisfactory, and the problems with them therefore interfere with the practical use of the fibers. Given this situation, there is a continuing need to develop highly-absorbent PVA fibers that solve the known problems and to prepare non-woven fabric prepared from such fibers.
Accordingly, one object of the present inventors is to provide PVA fibers of high water absorbency and that have good strength and elongation properties and from which fabrics containing the fibers can be readily prepared.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by cross-linked polyvinyl alcohol fibers prepared from a water-soluble polyvinyl alcohol, which satisfy the following requirements:
(1) a water absorption in water at 30xc2x0 C. ranging from 10 to 100 times the weight of the fibers;
(2) a fiber diameter in water at 30xc2x0 C. as a result of absorbing water ranging from 2 to 10 times the diameter of the fibers not having absorbed water; and
(3) a melting point ranging from 160 to 220xc2x0 C., and a heat of fusion ranging from 40 to 100 J/g.